Raw materials comprising a polymer compound have characteristic gas permeability for each raw material. Based on properties thereof, a desired gas component can be separated by allowing selective permeation, by means of a membrane constituted of a specific polymer compound. As an industrial application embodiment of this gas separation membrane, studies have been conducted for separating and recovering carbon dioxide from a large-scale carbon dioxide source, in a thermal power station, a cement plant, a blast furnace in a steel plant or the like, in relation to a global warming issue. Then, this membrane separation technique attracts attention as a solution to an environmental issue to allow achievement by relatively small energy. Meanwhile, natural gas or bio gas (gases generated by fermentation or anaerobic digestion of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbages, energy crops, or the like) is a mixed gas mainly composed of methane and carbon dioxide. Studies have been made so far for a membrane separation method as a means for removing an impurity, such as carbon dioxide therein (Patent Literature 1).
In regard to the purification of natural gas according to the membrane separation method, celluloses and polyimides have been studied as the materials for the membrane for use in the membrane separation method. However, due to the high pressure conditions in actual industrial plants, the influence of impurities present in natural gas or the like, the membrane is plasticized, and there has been a problem of a lowering in separation selectivity attributable to that (pages 313-322 of Non-Patent Literature 1, and Non-Patent Literatures 2 and 3). In order to suppress this plasticization of a membrane, it is known to be effective to introduce a crosslinked structure or a branched structure to the polymer compound that constitutes the membrane, and research has been conducted on a separation membrane which uses a polyimide resin (pages 3 to 27 of Non-Patent Literature 1, Non-Patent Literatures 4 and 5, and Patent Literatures 2 and 3).
For example, Non-Patent Literature 4 describes a gas separation membrane of a polyimide that is ionically crosslinked using a metal complex. In Non-Patent Literature 4, simple membranes having a thickness of 40 to 60 μm were produced using three types of polyimides such as a non-crosslinked form, a form crosslinked by covalent bonding, and an ionically crosslinked form, and the gas separation performance of those membranes were compared. It has been reported that an enhancement of the gas separation performance was almost not recognized in the gas separation membrane that uses an ionically crosslinked polyimide using a metal complex, and the gas separation performance was enhanced in the polyimide in which a crosslinked structure is formed by covalent bonding. Also, Non-Patent Literature 5 reports that aggregates of a metal complex are formed in the simple membrane that is ionically crosslinked using a metal complex as described in Non-Patent Literature 4.